Environmental influences on climate-driven growth of Mediterranean pollarded willows

Abstract

Anthropogenic woodlands dominated by pollarded trees are iconic landscapes common in Europe, where they represent unique cultural, aesthetic, ecological and economic values. A special type of these woodlands are riparian pollarded willows (Salix spp.), which show a strong coupling with shallow soil water sources and streamflow. Willows are fast-growing, pioneer species formerly used to obtain key resources for rural communities. However, the long-term responses of these species to pollarding abandonment and warmer and drier conditions are unknown, particularly in Mediterranean, seasonally dry areas. We also lack a good understanding on the diameter, age and growth rates of pollarded willows growing under different ecohydrological situations. Here, we sampled five willow (Salix alba) stands, located in north eastern Spain and growing under Mediterranean conditions, characterized by warm and dry summers, and analyzed them using tree-ring data. Lower-basin dry sites near intermittent streams and upper-basin cold-wet sites near permanent streams were compared. We compared these stands with two coexisting poplar (Populus nigra) populations. Growth in lower-basin sites located near intermittent streams was constrained by low precipitation and soil moisture from spring to summer, whereas growth in upper-basin sites situated near permanent streams and with deep soils was constrained by winter waterlogging, dry spring conditions and low summer temperatures. Willow growth declined in response to short-term spring-summer droughts. The different responses of Mediterranean pollarded willows to climate variables and river flow depend on their contrasting ecohydrological situations which condition their use and management.